DC tachometers, particularly those used in velocity servo systems, should ideally provide a smooth output signal proportional to speed without any ripple. Unfortunately, a significant ripple component is usually present at a frequency corresponding to the slot frequency, i.e., the frequency corresponding to the movement of armature slots past a particular point on the stator. To avoid instability and other problems in the servo loop, the bandwidth of the servo amplifier must be limited to safely reject or attenuate the ripple frequency in the normal operating range of the system. As a result, the frequency of the ripple component limits the frequency response of the servo system.
DC motors similarly have a ripple component in the output which appears in the form of a ripple torque in the motor output. If the motor is part of a servo system, the ripple torque will often affect the speed and, hence, be reflected in the velocity feedback signal for the servo loop. The ripple torque component, in general, is undesirable since it tends to create an undesired oscillation in the motor torque output.
An object of this invention is to provide a tachometer or motor in which the ripple component apppears in a form having less of an adverse effect than in previous dynamo electric devices.